LTE is a new radio access network technology evolved from 3G WCDMA providing high peak bit rates and good end-to-end QoS. Nevertheless, the wireless link is still likely to be the bottleneck of an end-to-end connection in many cases. In an overload situation, that is, a situation where the incoming data rate to the link is larger than the outgoing rate, the excessive data is temporarily stored in memory. This memory is often referred to as a transmit buffer or queue. If the overload continues, the data queue will accumulate and become large. This may cause a number of problems e.g., large end-to-end delays, unfair sharing between different flows etc.
Furthermore, because a buffer is finite, the queue might finally exceed the physical limitation, and some data has to be discarded. A straightforward way to handle this problem is discarding newly incoming data when the buffer is full. This approach is intuitive and easy to implement, however the performance is far from optimal in terms of end user delay.
A more sophisticated approach to managing buffer queues is referred to as Active Queue Management (AQM). AQM drops packets before the buffer is full. As a result, assuming IP packets are being sent over a TCP/IP link, the TCP sender can sense the segment loss and, as a consequence, reduce its sending rate—see Stevens, W. TCP/IP Illustrated, Volume 1: The Protocols. Addison-Wesley, 1994. In this way, the queue size and queuing time can be maintained at relatively low levels. Additionally, the throughput of the end-to-end link will not be reduced significantly.
Considerable work has been done for AQM. However, most AQM algorithms are designed for wired networks and are not suitable for mobile communication networks, because of their varying bandwidth characteristics, which result from varying radio conditions. Packet Discard Prevention Counter (PDPC) algorithm, on the other hand, is one AQM algorithm for WCDMA—see Sågfors, M. Ludwig, R. Meyer, M. Peisa, J. Buffer Management for Rate-Varying 3G Wireless Links Supporting TCP Traffic. In Proceedings of Vehicular Technology Conference, 2003. 3GPP also specified a simple algorithm called PDCP discard in its specification—see 3GPP TS 36.321. Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification, version 8.2.0. May 2008. Another AQM algorithm is proposed for 3GPP in 3GGP R2-080937, On the Need for Active Queue Management for non-GBR Bearers, TSG-RAN WG2 Meeting #60bis, February 2008. That proposal has not been accepted.
PDCP discard as specified for the UE in 3GPP Rel-8 is a simple delay-based algorithm which discards packets based on how long the packets have been in the PDCP queue. When the delay exceeds some predefined threshold, the packets will be discarded. This algorithm can maintain small end-to-end delay, but can result in considerable throughput degradation in some situations.
The more elaborate delay-based AQM algorithm proposed in the above-mentioned TSG-RAN meeting implements a mechanism to improve the throughput of PDCP discard, e.g., preventing consecutive packet drops, without introducing large end-to-end delay. However, as noted, the proposal was not accepted in 3GPP for LTE Rel-8. It would be possible to specify a UE based uplink AQM mechanism for LTE in Rel-9 (or later) to achieve a better performance but that would require standardization effort and the mechanism would not be available for Rel-8 UEs.
US2008/186918A1 relates to facilitated active queue management. The queue management can be effectuated in a serving radio base station as well as in the access terminal, and the application that generates the data packets can be executed locally or remotely to either the base station or the access terminal.
The 3GPP draft “SDU Discard”, R2-074689, Nov. 12, 2007 discusses AQM as a mechanism that controls the size of the L2 data queue. AQM is defined as a sender side function used to maintain the queue sizes on the sender side at an acceptable level, where a decision to dro a sacket can be based on e.g. the size of the queue. It is proposed that an AQM mechanism should be specified for LTE, and that the queuing should be modeled in PDCP and the AQM mechanism should be located in the PDCP layer.
The 3GPP draft “Specifying SDU Discard function for the UE”, R2-074703. Nov. 12, 2007 relates to the use of a SDU discard mechanism to realize an AQM mechanism as a sender side function for L TE UE to control the size of the queue on the sender side and corresponding queuing delays. AQM drops packets to force higher layer protocols to reduce their send rate, thereby leading to reduced queue size and reduced delays.